1. Technical Field
The present disclosure relates to optical components. More particularly, the present disclosure relates to graded index birefringent components.
2. Description of Related Art
Surface relief birefringent elements are described, for example, in WO-03/015424 and WO-2005/006056. A surface relief birefringent element is formed from a surface relief interface between an isotropic layer and a birefringent layer. Light of a first linear polarization state passing through the surface relief birefringent element sees a first refractive index step at the surface relief interface between the isotropic layer and the birefringent layer, whereas light of a second orthogonal linear polarization state sees a second, different refractive index step at the surface relief interface.
Surface relief birefringent elements can be formed by a liquid crystal cell filling method as shown in FIG. 1. A substrate 2, such as a glass or polymer substrate, has an isotropic polymer layer 4 formed on its surface by ultraviolet (UV) casting, embossing, thermal forming or other well known methods. An alignment layer 6, for example polyimide, is formed on the surface of the isotropic polymer layer 4, for example, by spin coating, printing or other known methods. The material of the alignment layer 6 is cured and rubbed to produce a directional alignment property. Another substrate 8 with another alignment layer 10 forms a cell gap between the alignment layers 6 and 10 which is capillary filled by a liquid crystal material 12 in a direction 14 typically at elevated temperature.
The liquid crystal material 12 is a curable liquid crystal material. In this case, following filling, the liquid crystal material 12 is cured, for example thermally, by light, such as from a UV lamp 28 or by electron beam radiation. Such materials allow high ruggedness and can enable a reduction in the thickness of devices.
Such a filling process has a number of difficulties. A lenticular surface with an array of elongate cylindrical lenses is a common surface relief interface. In this case, the capillary fill will often take place along the length of the lenses. However, these lenses may be susceptible to blockage, so that they do not fill uniformly, creating bubbles which degrade optical performance. In curable liquid crystal materials, bubbles may contain oxygen which inhibits cure of some types of polymerizable liquid crystal materials. This can cause regions of strain in the cured liquid crystal material, degrading alignment properties of the liquid crystal material near the bubbles. More even, filling can be achieved by incorporating a larger spacer gap between the alignment layers 6 and 10. However, such an approach disadvantageously uses more material and so increases cost. Further, the uniformity of the thickness of additional material can be difficult to maintain, so the final device may not be flat, which may cause non-uniform optical output, for example, in an autostereoscopic display system. During filling, a vacuum can be used to avoid the formation of air bubbles. Vacuum equipment is disadvantageously expensive, and the high levels of vacuum used for vacuum filling may not be compatible with the lens polymer materials.
The surface relief birefringent element of FIG. 1 requires the substrates 2 and 8. Such substrates typically have a thickness of 0.4 mm or greater. The overall thickness of the surface relief birefringent element is thus increased. To reduce thickness after fabrication, it may be possible to remove the substrate 8 from the cured liquid crystal material 12, but if this is glass, it may be prone to cracking. The substrate 8 may comprise a metal foil, which when removed still produces a transparent device. The surface energies of the interfaces between the cured liquid crystal material 12 and the alignment layers 6 and 10 may be similar, so that delamination may take place off either surface, therefore resulting in unreliability of delamination release. Further, the addition of alignment layer 10 adds cost to the processing method. Further, the adhesion of the cured liquid crystal material 12 to the alignment layer 6 is required to be as high as possible, to maximize the endurance properties of the surface relief birefringent element. Higher surface energy may be achieved by addition of a wetting agent to the liquid crystal material 12. However, this may also increase the adhesion to the alignment layer 10, and thus reduce the reliability of delamination at the planar interface between the liquid crystal material 12 and the alignment layer 10.
Further, the filling process can take some hours, particularly for a large cell required for large displays or for motherglass processing methods. Liquid crystal polymer materials may be liable to unwanted thermal cure prior to cure by, for example, UV radiation. This means that they are difficult to use reliably in processes with prolonged process time. Premature cure may result in regions of non-uniform liquid crystal alignment and filling errors.
Surface relief birefringent elements such as shown in FIG. 1 are difficult to cut after processing in motherglass form. It is required to cut through the glass substrates 8 and 2 as well as through cured polymer layers, such as the cured liquid crystal material 12 and the isotropic polymer layer 4, without causing delamination of the isotropic polymer layer 4 or the cured liquid crystal material 12.
The manufacture of uniform thickness birefringent elements using liquid crystal in solvent is described, for example, in U.S. Pat. No. 5,132,147, U.S. Pat. No. 6,262,788 and “Paliocolor polymerizable liquid crystalline acrylates for the manufacturing of ultrathin optical films” Cordula Mock-Knoblauch SID Digest 2006. In the latter, a coating solution comprises a liquid crystal polymer material in a solvent solution. The coating solution is applied wet to the surface of a polymer film. The solvent is driven off, and the liquid crystal polymer material is exposed to UV light to cure the film.
FIG. 2 shows a coating method for surface relief birefringent elements. A substrate 2 bearing an alignment layer 6 is overcoated by a coater, such as a slot coater 19 filled with a liquid crystal polymer material 12. Such an arrangement would be intended to produce a film of liquid crystal polymer material 12 in contact with air or a gas such as nitrogen. The film is cured by a UV lamp 28. If the prior art process were applied to the coating of surface relief birefringent elements with a curable liquid crystal polymer material, a number of difficulties would arise. The typical dry film thickness of the prior art planar devices is between 1 to 10 microns and requires the deposition of a wet thickness of 10 to 30 microns. The typical sag of the surface relief birefringent elements used in autostereoscopic displays is 15-60 microns, and so the thickness of typical cured films would need to be substantially more than that delivered by the prior art methods. Further, the flow of the drying material on the substrate 2 may not deliver a flat surface due to differential drying properties across the width of the substrate 2.
Additionally, the interaction of a single alignment layer 6 with the liquid crystal material 12 diminishes with increasing liquid crystal layer thickness. For relatively thick surface relief birefringent elements, it is desirable that an alignment layer is fixed on both sides of the liquid crystal layer. It is further often desirable that there is a controlled twist between the alignment directions at the planar and surface relief interfaces. In structures with a single alignment layer, a precise twist cannot be achieved as there is no upper surface to define an alignment. Further, the surface tension properties of coated microstructures can result in the upper surface assuming a non-flat structure, with different alignment properties at the cusps of the lenses compared to the center of the lenses. Such a structure will result in reduced optical quality. The elements need to be maintained clean during subsequent handling. Therefore, an additional protective cover may need to be added, further adding to the cost of the elements.
WO2008062188 describes a method to form a surface relief birefringent element using a coating apparatus as shown in FIG. 3. The structure comprising a substrate 2, an isotropic polymer layer 4, and an alignment layer 6 is mounted on a substrate 26, and a curable liquid crystal material 12 is applied by an applicator 11 to the surface of the isotropic polymer layer 4 and the alignment layer 6 while a film 16 with an aligning property is applied to the opposite surface of the curable liquid crystal material 12 by a roller 20 rolling in a direction 22. The cured film 17 is produced by applying UV light from a UV lamp 28 through the film 16. Such an arrangement provides a convenient method to fabricate a surface relief birefringent element with alignment features on surface relief and planar surfaces. Such surface relief birefringent elements have several advantages including that they are convenient to cut, have high optical quality with low thickness, do not suffer from premature thermal cure and are quick to coat compared to capillary filling methods.
The optical power of surface relief birefringent elements illuminated by a defined polarization state is limited by the curvature of the surface and the refractive indices of the birefringent and isotropic layers. The radius of curvature of the lens is typically constrained to be greater than half of the lens pitch. Further, the choice of refractive indices, particularly for curable liquid crystal materials may be limited.
It is sometimes desirable to increase the optical power of the surface relief birefringent element to a value greater than can be provided by a single surface and conventional liquid crystal materials. One arrangement is as shown in FIG. 4 in which a first surface relief birefringent element 50 comprising an isotropic polymer layer 4 and a cured liquid crystal material 12 on a substrate 2 is aligned to a second surface relief birefringent element 60 with respective substrate, isotropic polymer layer and cured liquid crystal material. Such an arrangement has a high thickness, particularly due to the isotropic polymer layer 4 and the substrate 2 and requires alignment of the two respective surface relief birefringent elements 50 and 60 thus having an increased cost.
FIG. 5 shows an arrangement to further increase the optical power of the surface relief birefringent elements in which four surface relief birefringent elements 50, 60, 70 and 80 are arranged in series. Such an arrangement has even greater thickness and cost.
FIG. 6 shows on-axis imaging of the arrangement in which on-axis rays 90 are imaged to a focus 92 at an image plane 94 (such as the pixel plane of an liquid crystal display (LCD) in an autostereoscopic display) by surface relief birefringent elements 50, 60, and 70 respectively. In this case, the on-axis rays 90 are correctly imaged by each surface to the image plane 94. Further, as shown in FIG. 7, for the example of a three surface relief birefringent elements stack, the separation of the surface relief birefringent elements 50, 60, and 70 means that the imaging properties of the arrangement for off-axis rays are degraded due to vignetting of light between apertures in adjacent surface relief birefringent elements 50, 60, and 70 in the stack. Substrates 2 are removed from the figures and three surface relief birefringent elements 50, 60, and 70 are shown for clarity. However, in the off-axis case as shown in FIG. 7, multiple spots 93 and 95 are produced at the image plane 94 so that the image produced by the surface relief birefringent elements stack is degraded. Thus, it is difficult to significantly increase the optical power using surface relief birefringent elements while maintaining image quality and minimizing cost.